fix(graph): stabilize debouncer tests with synctest fake clock

Replace real time.Sleep waits with testing/synctest fake clock to
eliminate CI flakiness caused by timer races on loaded machines.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

graph/debounce_test.go

@@ -3,6 +3,7 @@ package graph
 import (
 	"sync/atomic"
 	"testing"
+	"testing/synctest"
 	"time"
 
 	"github.com/stretchr/testify/assert"
@@ -10,6 +11,7 @@ import (
 )
 
 func TestDebouncer_Coalesces(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
 		d := NewDebouncer(50 * time.Millisecond)
 		var calls atomic.Int32
 
@@ -20,13 +22,16 @@ func TestDebouncer_Coalesces(t *testing.T) {
 			})
 		}
 
-	// Wait for the debounce delay plus some margin.
-	time.Sleep(150 * time.Millisecond)
+		// Advance fake clock past the debounce delay and let goroutines settle.
+		time.Sleep(50 * time.Millisecond)
+		synctest.Wait()
 
 		assert.Equal(t, int32(1), calls.Load(), "rapid calls should coalesce into a single execution")
+	})
 }
 
 func TestDebouncer_DifferentKeys(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
 		d := NewDebouncer(50 * time.Millisecond)
 		var calls atomic.Int32
 
@@ -34,24 +39,31 @@ func TestDebouncer_DifferentKeys(t *testing.T) {
 		d.Debounce("key-b", func() { calls.Add(1) })
 		d.Debounce("key-c", func() { calls.Add(1) })
 
-	time.Sleep(150 * time.Millisecond)
+		time.Sleep(50 * time.Millisecond)
+		synctest.Wait()
 
 		assert.Equal(t, int32(3), calls.Load(), "different keys should fire independently")
+	})
 }
 
 func TestDebouncer_TimerReset(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
 		d := NewDebouncer(100 * time.Millisecond)
 		var value atomic.Int32
 
 		// First call sets value to 1.
 		d.Debounce("key", func() { value.Store(1) })
 
-	// Wait 60ms (less than the 100ms delay), then replace with value 2.
+		// Advance 60ms (less than the 100ms delay) — first timer hasn't fired.
 		time.Sleep(60 * time.Millisecond)
+
+		// Replace with value 2 — resets the timer to fire at 60+100 = 160ms.
 		d.Debounce("key", func() { value.Store(2) })
 
-	// At 60ms the first timer hasn't fired yet. Wait for the second timer.
-	time.Sleep(150 * time.Millisecond)
+		// Advance another 100ms (total 160ms) to fire the reset timer.
+		time.Sleep(100 * time.Millisecond)
+		synctest.Wait()
 
 		require.Equal(t, int32(2), value.Load(), "later call should replace the earlier one")
+	})
 }